In a conventional optical disc system, to sense the position of the laser beam in relation to the track on the disc, the main laser beam creates a reflection from the disc. The reflection is typically picked up by 4 photo-diode sensors (or a photo detector array). FIG. 1 is a conceptual diagram illustrating how such a photo-diode configuration is laid out in relation to the track direction. The outputs of the 4 photo-diodes (when the laser beam is focused on the disc) are shown as signals A, B, C and D, respectively.
A track position (e.g., the location of a laser spot relative to a track center on an optical disc) is detected by imaging the laser spot on the photo detector array. Diffraction causes a slight change in intensity on the two different sides of the photo detector array when the relative position of the laser spot and center of the track changes. The difference in intensity on the two different sides of the photo detector array is called a push-pull signal. The push-pull signal is proportional to the tracking error signal or signal TE.
If the laser does not shine directly through a center of the lens, an image is moved to one side and the push-pull signal changes. Such an effect is defined as the center error (CE). The center error cannot be distinguished from the push-pull effect by examining one laser spot alone. To obtain an accurate track position, a second measurement is taken one-half track away from where the first measurement was taken. With the second measurement, the center error is common to the first measurement, but the push-pull effect is reversed. By combining the first and the second measurements, an accurate track position can be determined. For DVD ROMs, a phase detection method is used to detect the track position. The phase detection method is mostly immune to the effect of center error.
In an optical pick-up unit (OPU), the lens is held in position by springs in a sled housing. The optical center of the lens in the OPU is defined as the position of the lens where the center error is zero (i.e., where the laser is shining through the center of the lens). However, the position of the lens where the center error is zero, may not be the natural position of the lens when both springs are in a mechanical equilibrium state (e.g., when no control force is being applied). The natural position of the lens is defined as a mechanical center. Therefore, the center error is not necessarily zero when (i) the lens is at the mechanical center or (ii) the output of a lens controller is zero.
The motion of the laser spot is a superposition of the motion of the sled plus the motion of the lens inside the sled housing. A track seek initiated by the motion of the lens is called a fine seek (or fine seek mode for the system). The fine seek mode is slow because the laser spot remains locked to the disc even while the laser spot is crossing the tracks. The lens may move over several hundred tracks under the fine seek mode. However, if a target track is displaced at a large distance from where the laser spot is currently positioned, the lens cannot move fast enough under the fine seek mode. Therefore, the sled motor is used to reposition the lens under such a condition. A rough seek mode includes moving the lens and the sled housing with a sled motor to move the lens to the target track. While in the rough seek mode, the laser spot is unlocked from the disc. The signal CE is used to control or position the lens to the center of the housing. Positioning the lens to the center of the housing prevents the lens from inadvertently hitting the housing when the sled motor accelerates or decelerates in the rough seek mode. Such an impact can cause the lens to loose focus.
When it is necessary to lock the laser spot back on the tracks, a CE controller will switch to a tracking controller (or tracking error (TE) controller) which servos on a tracking error signal. When it is necessary to lock the laser spot back on the tracks, the center error signal will no longer be used to control the lens. Any previous control output to keep the lens at the optical center and not on the mechanical center will be lost when the control of the lens is switched from CE controller to the TE controller. The change in control from the CE controller to the TE controller will introduce a transient effect that affects the lock-on-track performance at the end of the rough seek mode. Conventional methods are characterized by an increase in seek time. The seek time includes the time for the lens-to-disc motion (which may be induced by switching from the center error signal to the mechanical center) to dissipate.